This invention relates to atomizers. In particular it relates to a rotary atomizing head for an atomizer of the type where a viscous liquid such as paint is atomized by a rotary atomizing head turning at high speed, to strike an object to be coated therewith. A specific utilization is in a rotary atomizing head for an electrostatic coating machine.
Heretofore, a so-called "mini-bell type rotary atomizing head" has been extensively employed as the above-described rotary atomizing head. This device comprises: a cup-shaped rotary atomizing head body (or a bell rim) which is mounted on the front end of a rotary shaft and has a conical inner surface with a circular outer edge. A part receives a stream of paint to be atomized (or a bell hub) provided in the atomizing head body. Japanese Patent Application Laid-Open Nos. 147740/1978, 47159/1980 and 47160/1980 and Japanese Utility Model Application Laid-Open Nos. 49797/1980 and 107255/1980 disclose the structure of such conventional rotary atomizing heads for atomizers.
In the above-described mini-bell type rotary atomizing head, the liquid paint supplying nozzle supplies liquid paint to the bell hub, where a paint film is formed by the centrifugal force which is created by rotation of the cup-shaped atomizing head. The paint film thus formed is supplied through the discharging passages to the conical inner surface of the atomizing head body.
However, the conventional rotary atomizing head has the following deficiencies when supplying liquid paint. When the rotary atomizing head turns at a speed of about 4,000 r.p.m. or higher, a negative pressure is provided in the space in the atomizing head, so that air is included into the space; that is, a so-called "air pumping phenomenon" occurs. In this operation, some of the water based paint droplets discharged from the rotating discharging edge stick to the inner surface of the atomizing head, going with the air which is induced into the space in the atomizing head. On the inner surface where no water based paint film is formed, moisture is evaporated from the droplets sticking thereto. As a result, the solid components are deposited. If, under this condition, water paint is atomized continuously for a long period of time, the solid components are successively deposited to grow vertically which finally obstructs the film flow of water paint. If atomizing further continues, droplets will stick to the discharging edge of the inner surface where the flow of water paint is obstructed. Hence, solid components are deposited, thus growing to the discharging edge.
Accordingly, while being guided, in the form of a film to the discharging edge, the water paint cannot be formed into a uniform film. Therefore, in the case where the atomizing head is used for a coating machine, the water paint droplets are large in diameter, thus lowering the quality of coating. Furthermore, in the case of a coating machine using an air motor, the speed of rotation of the atomizing head is decreased by solid components deposited, thus causing the same difficulty.
The paint supplying nozzle supplies water paint to a paint receiving part, where a water paint film is formed by the centrifugal force. The water paint film thus formed is guided to the inner surface of the atomizing head. In this operation moisture evaporates from the water paint in the solid-gas-liquid interface in the atomizing head and the solid components thereof are deposited in the interface. If, under this condition, water paint is atomized continuously for a long period of time, then the solid components successively deposited are mixed with the supplied water paint. Accordingly, the solid components are discharged from the discharging edge without being dissolved before reaching the discharging edge. Therefore, if the atomizing head is used for a coating machine, large masses of solid components may exist in the water paint droplets, thus lowering the quality of coating. In the case when the solid components have stuck to the inner surface of the atomizing head without being discharged from the discharging edge, the solid components are not dissolved in the water paint film.
Accordingly, it is difficult to form a uniform film on the discharging edge where the solid components have stuck, so that the droplets are stuck to the discharging edge of the inner surface where no film has been formed. As described above, the solid components are deposited in that location, thus lowering the quality of coating. The speed of rotation of the atomizing head is decreased by the solid components thus deposited, thus causing the same trouble.